LIDAR systems have been proposed for use in connection with various applications. For example, NASA and the scientific community have been working towards a space mission to characterize vegetation, specifically vegetation canopy. Data obtained from this mission would assist in trying to close the Earth's carbon budget as well as give a global baseline to vegetation health. In addition, LIDARs have been used in connection with altimetry and target identification. Other applications in space include rendezvous and docking, robotic rovers, planetary mapping, terrain relative navigation for future landers, and long range target position and range tracking.
An along track vegetation LIDAR design preferably has continuous coverage by the laser and a 25 meter footprint. For a LIDAR carried by a satellite, this drives a laser repetition rate of approximately 250 Hz. This is a relatively high repetition rate, and has a significant impact on laser reliability because of the high number of laser shots required for the mission (e.g., 10 billion). The high repetition rate also means that, in order to keep the average output power reasonable to improve laser reliability, the peak power per shot must be kept relatively low (e.g., 15 mJ). Accordingly, such a system may have insufficient power to make measurements through clouds and heavy aerosol layers.
A common issue for imaging LIDARs is that, in contrast to traditional imaging cameras, LIDARs must carry their own light source. Moreover, the performance and size/weight/power of imaging LIDARs are strongly dependent on the amount of laser light they produce. Trying to illuminate large scenes with laser light is challenging, especially at long distances. Any light that is collected and that falls below the detection threshold for a pixel is lost. Any light that misses the target is lost. This means that the laser light (photons) is of high value, and system designs must use the light as efficiently as possible.
One limitation of staring LIDARs has been their small cross track coverage. In particular, covering a large across-track swath on the ground using a staring LIDAR approach with multiple lasers is prohibitive in terms of available electrical power in space. In addition, LIDARs require that the transmitted laser spot be well within the receiver field of view over the target area. Large fields of view per detector are not desirable because a wide field of view results in the acceptance of more background light than a narrower field of view, reducing the signal to noise ratio. Within the field of view, the goal for good daytime performance has been to have the laser spot size only slightly smaller than the receiver's instantaneous field of view. This requires accurate boresighting of the LIDAR instrument. However, mechanical boresight mechanisms can be heavy and are expensive to produce and test.
In order to match laser illumination in a LIDAR to mission requirements, complex laser transmitter optics that shape or form the beam into patterns, or mechanical scanning systems, have been proposed. For example, in some mission scenarios, it may be desirable to attenuate the illumination signal, for example where a reflective target has entered the field of view of the LIDAR. As another example, widening of an illumination beam may be desirable when a LIDAR system used in connection with a landing system switches from an altimetry mode to a terrain relative navigation mode, and then to a hazard avoidance mode. Accordingly, attenuation and/or diffusion devices that can be selectively switched into or out of the illumination beam have been developed. However, such mechanisms typically result in the wastage of photons in at least some configurations, and introduce elements that can be subject to mechanical failure and/or misalignment. These systems also have the effect of shifting the effective dynamic range of the overall detector, rather than broadening that dynamic range.